There is an extra line at the end of the ckeditor.plugin file (after ?>) as included with FUDforum 3.0.5. This causes attachment files to be modified on download (0x0A added to the beginning of the file), such that image files are corrupted and cannot be opened (see http://fudforum.org/forum/index.php?t=msg&th=123401&start=0&).

After some further work, I'm not sure the blank line actually exists in the original file. I have a test and production instance. When I applied the ckeditor.plugin fix to the production instance, the download problem did not go away. I had to remove a blank line from a different plugin file (one I wrote myself). On the test instance this file did not have a blank line at the end.

In both cases the file needing to be fixed was the one listed last in cache/plugin_cache.